In January, at a US Space Force Base in the Rocky Mountains near Colorado Springs, an operator received an unusual email. It was an alert — but not one triggered by the military’s network of space-based sensors. Nor had it originated from the Space Force’s array of early-warning radar systems. Instead, the email appeared to have been forwarded to US Space Command by a tiny UN outfit in Vienna called the Office for Outer Space Affairs. The subject line read “Potential Asteroid Impact Notification”.

Upon scanning the email, the operator, according to one ex-Space Force official, began to panic. For the first time, the world’s alert system for defending the planet against incoming asteroids and comets had been activated. “We are,” said the anxious operator to a colleague, “under asteroid attack”.

The asteroid in question was 2024 YR4. Its name, according to the conventions of the International Astronomical Union, referred to the time of its discovery at the end of last year. At that point, 2024 YR4 was around 830,000 kilometres away from the Earth and orbiting the sun at a speed of 13km per second. Its brightness indicated it was between 40 and 90 metres long. It was rotating quickly, spinning once every 19 minutes upon its axis. It appeared to be gently elongated. From certain angles it looked like a fish head. The object was moving swiftly away from our planet, but when astronomers did a rough calculation of its orbit, they realised there was a slight chance that it might, in eight years, swing back and collide with the Earth.